Study On Mechanical Properties Of Low-activation Steels Based On Small-ball Punch Test

Low activation steels are the prime candidate materials for advanced reactors. The severe environment of high temperature, high press and intense irradiation inside the reactors leads to a strong demands for superior mechanical properties of materials candidate to the advanced nuclear reactors. High energy ions provided by HIRHL(Heavy ion Research Facility in Lanzhou) were used to simulate radiation effects of high energy neutrons. In order to study irradiation damage, a small specimen is required because of the strict limitations on specimen size in irradiated material testing facilities(due to the limited space available for testing in nuclear reactors and the narrow damage zone produced by charged particle accelerators). A small specimen test technique(SSTT) called small ball punch test(SP) was used in this work to acquire mechanical properties of specimens before/after irradiation. Finite element model(FEM) of SP was developed and used to analyze the stress-strain state in specimens.The phenomena of circumferential cracks observed on ductile materials were explained by FEM. It is indicated that the crack should be initiated at the bottom of the specimen. The calculated radial distance from the specimen center of circumferential crack was in line with the observation of SEM. Fracture surface of specimens obtained from mechanical test possess a fractal characterization. Digital images of the fracture surfaces obtained from scanning electron microscopy(SEM) were used to calculate the fractal dimension(FD) by using the pixel covering method with a MATLAB program. The results indicate that fractal dimension can be an effective parameter to describe the characteristics of fracture surfaces before and after irradiation. The rougher the fracture surface, the larger the fractal dimension. Correlation of the change of fractal dimension with the embrittlement of the irradiated steels is discussed.